In a statement, Monteith's rep confirmed Sunday night that the actor has "voluntarily admitted himself to a treatment facility for substance addiction.
"He graciously asks for your respect and privacy as he takes the necessary steps towards recovery."
People first reported the news on its website.
This is not the 30-year-old's first time in rehab. He got treatment when he was 19 and previously has spoken about his addiction struggles, saying he had a serious problem and took just "anything and everything."
He told Parade in 2011 that he was "lucky to be alive."
Monteith stars as Finn Hudson on the Fox show. He dates his co-star, Lea Michele. She told People that she loves and supports Monteith and is proud he's going to rehab.